You have your air stones too low in the water at a 3' depth. I belong to a large pond forum, and most folks there recommend no more than a foot deep during the winter. You want as much agitation towards the surface as possible.
I have a small raised garden pond (425 gallons). I raised my air stones and still have my submersible filter running. I did add a Sunbubble to gain solar heat during the day. This past week we have had unseasonably cold temps with minus and single digits during the nights and teens and 20's during the day. We are predicted to have some single digit days later this week. So far, I haven't seen a sliver of ice, and I am hoping it stays that way. I would hate to have to stick my hand and arm 30" down in frigid water to pull that filter.
EDIT: Just went outside and this is the first day I have seen ice, and it is about 1" thick, but the area above the air stones and the area where I have the submersible filter discharge pointed towards the surface are ice free. I guess the constant real low temps this past few days are catching up. I did smack the sunbubble's vinyl siding on the 3 southern facing panels to clear them; the panels gets a coat of frosty ice at night due to the warmer water condensate that hits it. Clearing them allows greater sun penetration. Looks like we're going to have a sunny day, and I want to take advantage of any solar gain I can get.
